W. K. WILSON,C. M. MASON, J. H. HICKEY AND J. H. MACK
VOl. 64
[CONTKIBUTION FROM THE CHEMISTRY DEPARTMENT O F THE UNIVERSITY OF NEW HAMPSHIRE]
The Magnetic Rotation of Cerium Salts in Aqueous Solution BY WILFRED K. WILSON, CHARLES iM.MASON,’ JOSEPH
Within recent years there have been several investigations of the magnetic rotation of cerous salts in aqueous solution. ‘The nitrate has been investigated by 13llai,” and by Clhuckerbutti,s and an investigation of the sulfate has been made by Roberts, Waliace and Pierce.8 Previous investigations of the magnetic rr)tation of the chloride have been carried out by Pernet* and Slack, Reeves and Peoples.5 In the course of our general study of the properties of the rare earth salts in aqueous solution, an attempt was made to extend the work of these authors on cerous chloride. Since preliminary work indicated a divergence between the results previously reported and those found a t Ken. Hampshire, an extensive investigation has been made of the magnetic rotation of cerous chloride in aqueous solution.
Experimental The measurements were carried out as previously describeda except that additional wave lengths were obtained by use of a Pointolite lamp and a carbon arc in conjunction with the monochromator. Cerous chloride from two sources was investigated i n order to eliminate any error inherent in the source. A t first thought, it would seem that chemical analysis should be the .first and most important proof of purity but when it is considered that the impurities would most likely be other rare earths, some means must be found to supplement the chemical analyses. First, the magnetic rotation itself should serve as a means of comparing the ccrium from the two sources and, second, the Bureau of Standards kindly supplied us with a sample of cerium material whose spectroscopic analysis was known and this could be compared spectrographically w i t h the material to he used in the inves tigation. Two solutions were prepared by methods previously described,6 and compared as outlined herein. As one means of analysis, the two samples were coinpared by emission and absorption spectra against the sample procured from the Bureau of Standards. I n so far as could be determined the samples prepared were spectrographically equivalent t o this sample. As a second test for purity, we analyzed stock solutions from the two sources for their cerous ion content, and by a 11) Present address:
?’
S Bureau
of X i n e s .
University. Ala.
barns. (13) Pillai, I n d i u i i J. Phi..., 7, 8; i93z: ( 2 ) Chuckerbutti, i b i d . , 8, 387 (1933). S) Roberts, Wallace and Pierce, Phil. S f a d . , Sei 7. 17. vY4 11934). :4) Peruet, C o m p f . rend., 196, 376 (1932). t6) Slack, Reeves and Peoples, Ph53s. Rev., M , 724 (1934), [ti) Nasoli. Hickev and Tiison, Tare JOCRKAL, 63,95 (lv4U) I
w. HICKEY A S D JOHN
H. MACK
different procedure for their chloride ion content. If out compounds were chemically pure, the concentration of the solutions calculated from these two different ion content5 should check within experimental error. The method of analysis used for the cerous ion content involved the precipitation of the cerium as the oxalate and igniting it to the oxide under conditions outlined by Sarver and Brinton’ and Brinton and Pagel.8
Table I contains the results of these analyses, the concentrations being expressed in grams of cerous chloride per gram of solution. In each case the concentration cited is an average of three values. It can be seen that solution 1 gave values differing by about one part in a thousand, whereas the values for solution 2 differ by about three parts in a thousand. TABLE I Concentrnbon I 2
0.4042 0.4037
From chloride ion determination From cerous ion determination
0.3499 0 3390
These values check within experimental error and therefore as far as chemical analysis is concerned the solutions seem to be chemically pure, solution 1 being more pure than 2 . For this reason solution l was used in determining the Verdet constants which were the main object of this research. However, a third test was a t our disposal, that of magnetic rotation itself. If these two solutions were pure samples of cerous chloride, they should give the same rotation. I n Table I1 are the results for the magnetic rotation of 2.00 molal solutions prepared from the stock solutions. The values were taken over a range of temperature and wave length in order to give a more rigorous comparison. TABLE I1 Tl mp , o c
20
50
Wnve length,
A
6438 5461 4678 6.138 5461 46%
Rotation in degrees 1 2
-1.74
-2.90 -4.84 - 1 34 -2.28 - 4 03
-1 80 -2.93 -4.83 - 1 38 -2.31 -4 04
Considering that the PI etision of the polarimeter alone is 0.02’ €or one iotation, the values in Table I1 stand in fairly good agreemeiit except (7) Sarver and Briuton, %b%d.,40, 943 (1921). , 46, 14GO 11323!.
( 8 ) Brinton and Pagtl, ibid
Deb., 1942
MAGNETIC kOTATIbN OF
in the case of 20" and 6438A. Even this discrepancy becomes less significant when one considers the difficulty of taking readings in the red end of the spectrum, due to the lack of intensity and the fact that the eye is not particularly sensitive to that region of the visible spectrum. From these four tests of the purity of our samples, we felt warranted in making the conclusion that our samples were essentially chemically pure, solution 1 being more pure than solution 2, because the results from chemical analysis gave a better check. Therefore, we used solution 1 in all the work in refractive indices and Verdet constants that appear later in this investigation.
Results The relationship of the magneto-optical rotation to the field strength and length of the light path is expressed by Verdet's law9 r = VHl cos a
r = V f H d l
(3)
where f H dl is the average field strength over the light path used. If we choose to calibrate our cell with some pure liquid whose Verdet constant is known, such as water,l0we find that V = rV8/r8
Wught molality
V X 4678A.
(4)
where rs and V, are the rotation and Verdet constants of the standard liquid used, and r and V are those for the solution being measured. The Verdet constants of the cerous chloride solutions were obtained by measuring the magnetic rotation of the various solutions and then computed by means of eq. (4). The results so obtained are given in Table 111. Although the Verdet constants in the following table are in units of minutes per oersted-centimeter, they can be converted to M. K. S.I1 units of minutes per M. K. S. oersted-meter by multiplying by the proper factor. (9) E. Verdet, Ann. chinr. phys., 41, 370 (1854); 48, 87 (1854); 62, 129 (1858); 69, 415 (1863).
(10)Rodger and Watson, 2. physik. Chcm., 19, 356 (1896). (11) Jauncey and Langsdorf, "M.K.S. Units and Dimensions," The Macmillan Co., New York, N. Y., 1940.
101 (minutes per oersted-centimeter)
6086A.
546lA.
5893A.
643SA.
0.50 -25 .OO
-43.82 -37.61 -25.93 -14.30 3.05 9.64 15.80 21.53
2.753 2.50 2.00 1.50 1.00 0.50 .25 .OO
-42.80 -36.59 -25.18 -13.34 2.14 10.18 15.86 21.53
2.753 2.50 2.00 1.50 1.00 0.50 .25
.OO
-41.73 -35.57 -24.43 -12.96 1.39 10.29 15.96 21.48
2.753 2.50 2.00 1.50 1.00 0.50 .25 .OO
-39.75 -34.02 -22.93 -11.46 0.59 10.66 16.18 21.43
At 20" C. -33.96 -27.16 -28.87 -23.04 -19.55 -15.54 -10.39 7.82 0.69 0.00 8.48 7.55 13.34 11.52 18.00 15.48 A t 25" C. -33.05 -28.46 -28.18 -22.45 -18.80 -14.79 9.75 7.39 0.37 0.21 8.68 7.77 13.39 11.62 18.11 15.48 At 30" C. -32.14 -25.66 -27.48 -21.80 -18.11 -14.20 - 9.27 6.91 - 0.05 0.54 8.89 7.98 13.45 11.73 17.95 15.32 A t 40" C. -30.64 -24.27 -25.93 -20.57 -16.77 -13.07 8.20 6.11 0.43 0.96 9.37 8.14 13.61 11.95 17.84 15.27
2.753 2.50 2.00 1.50 1.00 0.50 .25
-37.98 -31.93 -21.59 -10.39 0.32 11.14 16.29 21.32
A t 50" C. -28.87 -23.14 -18.64 -14.04 -24.54 -19.50 -15.59 -11.95 -15.64 -12.21 - 9.74 7.18 7.39 - 5.30 4.07 - 2.79 1.07 1.66 1.82 1.87 9.05 8.46 7.39 6.43 13.77 12.05 10.18 8.62 17.73 15.16 12.86 10.66
2.753 2.50 2.00
1.50 1.00
-
-
-
(2)
When an electromagnet of less than infinite length is used, this becomes
413
TABLE111 VBRDETCONSTANTS OF CEROUS CHLORIDE SOLUTIONS
(1)
where r is the rotation in minutes, V is the Verdet constant, H is the field strength, 1 is the length of the light path in cm., and Q! is the angle between the path of light and the axis of the magnetic field. For the Faraday effect where the axis of the magnet and the rays of light are parallel, this law reduces to r = VHl
AQUFCOWS CERIUM SALTS
.OO
-
-
-
-
-
-
-
-
-
-
-21.86 -17.04 -18.59 -14.20 -12.21 - 9.32 - 5.95 - 4.34 0.32 0.75 6.59 6.00 9.86 8.41 13.07 10.93 -21.27 -16.39 -18.00 -13.82 -11.73 8.89 5.73 4.07 0.59 0.96 6.75 6.05 9.91 8.46 13.07 10.87
-
-
-20.73 -15.86 -17.57 -13.55 -11.30 8.46 - 5.30 - 3.75 0.86 1.18 6.91 6.11 9.96 8.52 13.02 10.76
-
-19.66 -14.89 -16.45 -12.59 -10.45 - 7.82 4.55 3.16 1.39 1.55 7.18 6.27 10.07 8.57 12.96 10.71
-
-
-
-
The refractive indices which were obtained are expressed in terms of linear equations of the type 71 = vo am bm2where 7 is the refractive index, qo the refractive index of water, a and b are constants, and m is the molality. The values of the constants of these equations for refractive indices at 25' are given in Table IV, and the refractive indices reproduced by these equations are within the experimental error, namely *0.0002.
+ +
W. K. WILSON,C. M. MASON,J. €3. HICKEYAND J. H.MACK
414
Vol. 64
The refractive indices for temperatures from 20 to 40' can be calculated using the equation qt = 73.6 - (t - 25)F where Tt is the refractive index a t temperature t , 7726 is the refractive index a t 25", and F is the temperature coefficient, given in Table IV.
a reasonable estimation would place the total possible error in Verdet constants a t f 0.3 X lo+. In view of this precision, it would seem that our results differ decidedly from the results of Slack, Reeves and Peoples. As would be expected, the difference increases with concentration of the solution, amounting to about 15% in the most conTABLE IV CONSTANTS FOR THE EQUATIONS FOR THE REFRACTIVE centrated solution. It hardly seems possible that INDICES OF CEROUS CHLORIDE SOLUTIONS AT 25" this difference is due to the impurity of OUT sarnWave ples or to error in our analyses when they were length, A. 170 a b checked and rechecked so repeatedly. If our work q = TO am bm2 is correct, then the deviation could only be ex4678 1.33778 0 04806 -0 00300 - ,00298 1.33564 .04757 5086 plained by some error in the analysis of their solu.04727 - ,00297 5461 1.33412 tions, the error probably lying in the incomplete 5893 1.33265 .04686 - .00296 ignition of the oxalate. They mention that their .04655 - ,00294 6438 1.33105 solutions had a brown color, even a precipitate qt = vzs - ( t - 25)F in some cases. It may well have been that a reaMolality 2.753 2.50 2.00 1.50 1.00 0.50 0.25 0 00 sonable concentration of reddish-brown ceric F X lo4 1.47 1 4 5 1.42 1.38 1 3 3 1.27 1 24 1 20 ions was present, for a pure solution of cerous chloIt is interesting to see how the data and sub- ride is water white. It might be added that the sequent calculations advanced in this paper com- solutions used in this work were water white, and pare with values found by previous investigators. even after standing a year there had been no preAs has already been mentioned, conflicting cipitation or discoloration. values for the Verdet constant of cerous chloride Discussion solutions already have been determined by Okazaki' has published a series of papers on the Slack, Reeves and people^.^ Table V will serve Faraday effect in solutions of electrolytes. He to show how their values compare with those of has used the following modification of the Schonthis research. The values submitted here as rock mixture rule being taken from Slack, Reeves, and Peoples5 (?.I = h ( W J t d W J (5) are actually averages of their Verdet constants at where w, wl, and 7 0 2 are the specific rotations of 30" and 30 O. This interpolation is entirely permissible, for they reported a linear relationship the solution, solvent, and solute respectively, and between temperature and Verdet constant, other tl and t2 are the weight percentages of the solvent and the solute. The molecular rotation, M ( w ) , iactors being constant. may easily be obtained by multiplying the specific TABLE V rotation by the molecular weight. We have used Wave Verdet constant X 103 a t 25' this formula to compute the molecular rotation Molality length, A. S R &P This paper -24.0 -21.2 2.753 5893 of the solute, M ( w ~ a) t 25'. The values so com-29.6 -26 1 5461 puted are included in Table VI. The densities 2.077 5893 -14.0 -13 5 used were taken from previous work. l 3
+
+
+
5461 1.087
5893 5461
-17.3
-15 9
- 0.58
-
-
1.15
0.4
- 1 1
Since the precision of the polarimeter is k0.01 O for any one reading, the precision of any rotation is naturally *0.02'. Since all rotations were multiplied by a cell constant of 0.005357, the precision of all Verdet constants obtained using that cell would be of the order +0.11 X lowJ. This precision does not take into consideration the contribution of errors due to temperature, current, and analytical discrepancies. Perhaps
TABLE VI OF CEROUS CHLORIDE MOLECULAR ROTATION Weight molality
2.753 2.50 2.00 1.50 1.00 0.50
0.25
-
46788.
508d.
24.9 24.5 24.2 23.7 23.8 22.6 22.5
19.8 19.5 19.1 18.9 18.6 18.8 18.7
M(w9) 5461A.
16.2 16.0 15.6 15.5 15.3 15.4 15.3
5893A.
6438A.
13.2 13.1 12.8 12.7 12.5 12.6 12.5
10.5 10.4 10.2 10.1 9.9 9.6 9.5
(12) A. Okazaki, Roqjus COX Eng., Mem., 6, 181 (1933); ibid.. Iuouye Commemoration Volume, 209 (1934); i b i d , 9, 13, 101 (1936); 10, 89, 115 (1937); 12,33,45 (1939). (13) Mason, THIS JOURNAL, 60, 1644 (1938).
Feb., 1942
MAGNETIC ROTATION OF
An examination of Table V I shows that the mixture rule (eq. 5) when applied to the data in Table I11 holds within limits of experimental error a t concentrations below about 3 molal which remesents about 40% anhydrous salt in the solutions. There are some discrepancies in the more dilute solutions but in this region where the rotation is small the errors of measurement are magnified in the computation of the Verdet constant and further magnified in the computations of the molecular rotation. These results in general confirm and are consistent with the results obtained by Okazaki12for many other electrolytes. Roberts, Wallace and Pierce3 have calculated for cerous sulfate solutions a quantity A/W which they call the "specific rotation." This is defined in the equation
where n is the refractive index of the solution, n, that for water, and W is the number of grams of salt per 100 g. of solution. V is the Verdet constant for the solution and V , is that for water. From the theory from which this equation is developed, the quantity A/ W should be independent of the concentration for any given wave length. This constant is independent of the concentration for cerous nitratee and an examination of the data given in Table VI1 will show if this be true for the salts studied in this investigation. Using the equation of Roberts, Wallace and Piercea the corrected specificrotations ( A / W ) were calculated a t 25" and the results are given in Table VII.
Weight molality
2.753 2.50 2.00 1.50 1.00 0.50 0.25 Average
TABLE VI1 CORRECTED SPECIFIC ROTATJONS - A / W x 103 I
?
46788.
50868.
54618.
5893B.
64388.
8.93 8.86 8.87 8.81 8.98 8.65 8.68 8.83
7.12 7.07 7.02 7.05 7.02 7.20 7.24 7.10
5.84 5.81 5.76 5.79 5.81 5.90 5.93 5.82
4.79 4.74 4.73 4.77 4.75 4.84 4.86 4.78
3.81 3.79 3.77 3.79 3.78 3.69 3.71 3.76
According to the theoretical background from which A/W was obtained, A / W should vary with the inverse square of the wave length. A plot (Fig. 1) shows this variation. This figure also shows the molecular rotation plotted as a function of the inverse square of the wave length. In both cases a linear plot was obtained within the
AQUEOUSCERIUM SALTS
415
experimental error. Recourse to Table VI1 however shows that A/W is not as constant as predicted by Roberts, Wallace and Pierce.
2.4
3.0
4.0 108. Fig. 1.-The variation of the calculated specific and molecular rotations with the reciprocal of the wave length. 1/x2
x
Although there seems to be some tendency toward variation in the solutions, in general the constancy is pretty good. It might be noted here that although Roberts, Wallace and Pierce did find A/W to be constant for cerous sulfate, the solutions they investigated were less than 0.50 molal. Whether that constancy would have extended to more concentrated solutions will remain unknown, for cerous sulfate is relatively insoluble. It is also worthy of note that the molecular rotations in Table VI show an effect somewhat the opposite to the variation in the corrected specific rotations. The values are essentially constant a t low concentrations, but vary increasingly a t higher concentrations. This effect might well be due to a variation from the mixture rule a t higher concentrations. Mlle. Pernet4 has discussed the shape of the curves obtained from her work on the magnetic rotation of cerous chloride solutions, but she does not show these curves nor give sufficient data to construct them. However, she does state that for the mercury green line (5461A.) zero rotation was obtained for a solution of 0.99 molality a t 22.6O. Interpolation from a graph of our data will yield a Verdet constant of 0.40 for the same conditions at 25". For that concentration value and wave length, the temperature coefficient for the Verdet constant is +0.50 per 10" centigrade, as interpolated from temperature variation for 1.OO molal solutions a t that wave length. Such a
416
W. B. WILSON,C. M. MASON,J. fi. HICKEYAND J. & MACK I.
variation would give a Verdet constant of 0.15 for 0.99 molal solution at 22.6’ and a wave length of 5461A. This value agrees with Mlle. Pernet’s within experimental error. But this agreement has no great significance,for similar agreementwith Mlle. Pernet was reported by Slack, Reeves and Peoples. How workers who disagree widely elsewhere could agree with Mlle. Pernet and thus with each other is easily explained. The “Pernet Point” occurs in a concentration whose temperature coefficient is relatively small, and naturally any slight difference in concentration or temperature would be minimized there.
P X
b
Fig.
It is apparent, as shown in Fig. 2, that there is a straight line relationship between concentration and Verdet constants of cerous chloride solutions. A straight - line relationship between Verdet constant and temperature, other factors being
Vol. 64
constant was also obtained and it was apparent from these curves that the temperature coefficient (slope of curves) was greater in the more concentrated solutions. We wish to thank Dr.William 34’.Meggers and the U. S. Bureau of Standards for supplying us with cerium material of known purity and Dr Albert F. Daggett for assistance in the spectrographic analysis.
Summary and Conclusions Two samples of cerous chloride have been prepared and found to be of a high degree of purity. The magnetic rotation of cerous chloride in aqueous solutions has been determined over a wide range of temperature and concentration. The refractive indices of cerous chloride solutions have been determined over a wide range of temperature and concentration. It wits found that the Verdet constants of cerous chloride solutions have a straight line relationship with concentration, other factors being constant. Also, there is a straight line relationship between Verdet constant and temperature. Molecular rotations based on the Schonrock mixture rule are found to be strongly negative and show some variation with concentration, but are essentially constant a t low concentrations. Corrected specific rotations ( A / W ) based on the equation of Roberts, Wallace and Pierce3 also show some variation with concentration but are essentially constant at high concentrations. Furthermore, we have been unable to verify the values of Slack, Reeves and people^.^ In view of the admitted discoloration of their solutions, and in view of the care taken in this work to test the purity of the cerous chloride used, it seems that a well-substantiated contradiction of their work is to be found in this report. RECEIVED AUGUST16, 1941 DURHAM, N. H.
